Surveying is used for identifying subterranean elements, such as hydrocarbon reservoirs, freshwater aquifers, gas injection zones, and so forth. Surveying can include seismic surveying or electromagnetic (EM) surveying. In seismic surveying, seismic sources are placed in various locations above an earth surface or sea floor, with the seismic sources activated to generate seismic waves directed into the subterranean structure.
The seismic waves generated by a seismic source travel into the subterranean structure, with a portion of the seismic waves reflected back to the surface for receipt by seismic receivers (e.g., geophones, hydrophones, etc.). These seismic receivers produce signals that represent detected seismic waves. Signals from seismic receivers are processed to yield information about the content and characteristic of the subterranean structure.
EM surveying involves deployment of one or more EM sources that produce EM waves that are propagated into the subterranean structure. EM signals are affected by elements in the subterranean structure, and the affected signals are detected by EM receivers, which are then processed to yield information about the content and characteristic of the subterranean structure.
Surveying of a subterranean structure can be performed in a marine environment, in which a marine vessel tows survey sources (e.g., EM sources or seismic sources) for generating survey signals to perform the survey. Survey receivers are used for measuring responses of the subterranean structure to the survey signals. The survey receivers may be dropped to the water bottom surface (e.g., sea floor), or the survey receivers may be part of a streamer towed by the marine vessel.
For accurate analysis of results acquired by the survey system, it is desired to know actual positions of the elements of the survey system, including survey sources and survey receivers. To determine positions of the elements of the survey system, positioning networks can be deployed. A positioning network can be implemented with an acoustic positioning system, for example, which includes acoustic transmitters and receivers that send acoustic signals to allow for acoustic positioning of target elements of the survey system.
Typically, an acoustic positioning network is outfitted with a front buoy (float) and a rear buoy (float) that each includes a GPS (global positioning system) receiver. Positioning equipment is provided between the front and rear buoys. The GPS receivers on the front and rear buoys allow absolute coordinates of the front and rear buoys to be known, so that the positioning network can accurately determine positions of components of survey equipment.
However, buoys with GPS receivers are difficult to deploy, retrieve, and maintain. In particular, such buoys hinder streamer maintenance and stacking operations. Also, the buoys are subject to various failures, such as failures of data communication, power loss failures, and so forth.